At least one species of teleost fish uses a form of echolocation to detect the presence of nearby obstacles. The sea catfish (Arius felis) uses low frequency sound pulses for obstacle detection as well as in social communication. By means of behavioral techniques, the hearing capacities of this species will be determined, particularly those parameters that are necessary for echolocation, i.e., signal/noise ratio, frequency discrimination, and ability to determine the direction of a sound source. Fish have essentially two sensory modalities for audition: the lateral line and the swim bladder - inner ear complex. Behavioral techniques combined with surgical and chemical ablations will be used to study the relative roles of the two sensory systems in sound directionalization and echolocation. In this species, the male parent incubates the eggs through stages past hatching and until embryonic yolk is absorbed. Embryos and hatchlings can be maintained in the laboratory. Thus far, sound production has been observed shortly after hatching. The development of sonic behavior and its functions (social and echolocating) will be observed and recorded. Concurrently, specimens will be used for the study of the morphology of the sonic mechanisms and the auditory receptors. In particular, the orientation of hair cells in the inner ear could explain the unique acoustic capacities in this species. Echolocation in the sea catfish utilizes low frequency sounds (about 100 Hz), and the emitted sounds become mixed with reflected sounds, so that the echolocating system is judged primitive, as compared to the systems in dolphins and bats. This simple system is closer in its fundamental operation to the natural echolocating abilities that exist in human beings. The study of primitive forms of echolocation could provide significant information on the human capacities for such acoustic orientation.